Gas Chromatography - Tandem Techniques - The Basic Principles of Mass Spectroscopy > Page 34

 

After passing through the flame the light then passes into the optical system of the spectrometer. The flame burns a combustible gas mixture (e.g., air/acetylene, nitrous oxide/acetylene, air/propane or air/butane) in normal operation and the sample, dissolved in a suitable solvent, is nebulized and sprayed into the gas stream entering the base of the burner. When coupled with a gas chromatograph the column eluent is passed directly into the gas supply at the base of the burner. After passing through the flame, the light is either focused directly onto a photocell or onto a diffraction grating by means of a spherical mirror.

The diffraction grating is movable, and, thus, it can be adjusted to select a particular wavelength that is characteristic of the element being measured. Alternatively, with a dispersed source, the light can be scanned to produce a complete absorption spectrum of the sample. The position of the diffraction grating determines the wavelength of the light that is to be monitored. The flame absorption spectrometer is fairly sensitive, and can be readily combined with a gas chromatograph, providing an appropriate interface is employed. Gas chromatography/atomic adsorption spectroscopy is almost exclusively used for element identification and has been employed extensively in element speciation, pollution studies and forensic testing.

 

 

The Basic Principles of Mass Spectroscopy

 

Mass spectrometry, as opposed to the other spectrometric methods that have been discussed so far, is not involved with the absorption of electromagnetic radiation and, consequently, is an entirely different type of spectroscopic technique. Basically, mass spectrometry involves first, the production of ions from the sample eluted from the column. The ions can be molecular ions, ion fragments or ion complexes, depending on the type of ionization process that is employed. Second, having produced the ions, they are then accelerated to high velocities in a vacuum and, by arranging for them to pass through either electric or magnetic fields (or both) the ions can be separated from one another on the basis of their individual masses. By means of a suitable scanning procedure, each individual ion mass is then sensed and its mass identified. The advantages of this type of analytical approach and its value combined with a GC instrument are very obvious.

 

There are three basic types of mass spectrometer, the sector mass spectrometer, the quadrapole mass spectrometer (which includes the mass analyzer) and the time-of-flight mass spectrometer. All three types of mass spectrometer have been used (and, indeed, are still used) in combined configurations with gas chromatographs. It follows, that the basic principles of all three types of mass spectroscopic systems will need to be described. It should be noted, however, that the quadrapole mass spectrometer in one of its various forms is by far the most popular mass spectrometer to be used in a combined system, but the function of the sector instrument is the simplest to understand.